* In vivo and in vitro generation of Th2 cells in the absence of IL-4 receptor/Stat-6 signaling. In last year?s report, we described experiments demonstrating that the development of egg granulomas and fibrosis is ablated in schistosome-infected mice deficient in the receptor for IL-4 or its signaling element STAT-6 (Jankovic et al, JI, 1999b). Surprisingly, although these animals cannot respond to IL-4, the cytokine previously described to direct Th2 CD4+ lymphocyte development, they nevertheless produced significant levels of the lymphokines (IL-4, IL-5, IL-13) characteristically synthesized by Th2 cells. This year we formally demonstrated by single cell FACS analysis that significant numbers of conventional CD4+ lymphocytes with a classical Th2 phenotype (IL-4 , IL-5 positive; IFN-gamma , IL-2 negative) do indeed develop in IL-4R or Stat6 deficient mice in response to S. mansoni infection. While an expanded population of Th1 (IL-4, IL-5 neg, IFN-gamma, IL-2 pos) lymphocytes was also observed in the same animals due to the absence of cross-regulation, surprisingly, cells with a mixed Th0 cytokine pattern were present only in low frequency. Moreover, the cytokine production phenotypes of the Th1 and Th2 subpopulations generated in infected Stat-6 deficient mice were shown to be stable upon single-cell cloning and were unaffected by in vitro neutralization of endogenously secreted IL-4 or IFN-gamma. Finally, similar Th2 and Th1 populations could be generated from uninfected Stat-6 KO mice by repetitive polyclonal stimulation of na?ve CD4+ lymphocytes in vitro arguing that helminth infection is not uniquely required as a stimulus for this process. Taken together, the results of the above single-cell analysis indicate that CD4+ T cells with a Th2 lymphokine pattern can be generated spontaneously without the need for instruction through IL-4R/Stat-6 signaling. * Altered Th2 responses to schistosome eggs in mice deficient in Tec kinasesThe tec kinases rlk and itk are tyrosine kinases which play an important role in lymphocyte signaling. In a previous study (Shaeffer et al, Science, 1999) we showed that mice doubly deficient in both Tec kinases show markedly impaired Th1 cytokine responses and resistance to infection with Toxoplasma gondii. In order to assess Th2 response development in the same animals, we sensitized them by intraperitoneal injection with S. mansoni eggs and 2 wk later challenged them intravenously with eggs. Granuloma volumes in lungs and Th1/Th2 cytokine production in the draining lymph nodes were then measured. Consistent with their failure to develop Th1 immunity to T. gondii, the rlk-/ itk- mice displayed robust granuloma formation and highly polarized Th2 responses with no detectable IFN-gamma synthesis. Interestingly, singly deficient itk- mice showed markedly impaired granuloma formation and as expected a more Th1 skewed lymphokine response while rlk- deficient mice behaved like wild-type control animals. Since T lymphocytes from rlk-/itk- mice have been shown to be hyporesponsive to TCR stimulation, these observations reveal that conditions of weak TCR signaling can promote Th2 response development. - Schistosomiasis, immunopathology, CD4+ T cell differentiation, IL-4, Stat-6, Tec kinases, TCR signaling.